The choice depends on your reason for learning it. If you are wanting to have more options in business, you need to know which languages are used around the world for whatever business you are in. Good worldwide languages at present - those that cover a lot of different territory - would be Spanish, French, Mandarin Chinese, Japanese, Russian, Arabic, and Hindustani.

If you are learning it for pleasure, you might want to consider whether you want something easy or challenging. For the easy choice, select a language related to your own mother tongue (for example, English-German-Dutch-Norwegian-Swedish-Danish would be one group; Spanish-French-Italian-Portuguese-Romanian would be another; Slavic and Semitic languages would form other groups).

For a challenge, do the reverse and choose a language that is not related to your own. Some people even like to learn languages that few people know, in an attempt to help preserve languages that are dying out.

If you know a lot of native speakers of a particular language, you should learn that language. You will be able to practice speaking with them, so you would probably learn it far more easily.

If you don't, then you could decide which based on how commonly spoken a language is, the ease of finding learning materials, the ease of learning that particular language, a country you would like to visit or economic value.

The most widely spoken languages are Mandarin, Spanish, Hindi, Arabic and Bengali. Mandarin, Spanish and Arabic have a lot of economic value but Spanish is the only one that would be fairly easy to learn for an English speaker. Plus, you will find a huge amount of learning materials. There are so many Spanish speaking countries to visit and so many Spanish speakers in the US. For those reasons, I would suggest Spanish.